The project seeks to reveal the foundations for language development, by tracking adjustments of perception and production that infants make to the properties of their language environment. The first section of the project will employ native and nonnative phoneme contrasts to examine the young infant's perceptual sensitivities to speech sound distinctions, and how these become developmentally attuned to native phonological categories. The second section focuses on language-specific attunement in the development of speech production. The proposed studies will look for the ontogenetic emergence of language-specific prosodic and segmental properties in cross-language recordings of infant vocalizations, and in young language-using children's imitations of native speech. The experiments in the third section will investigate the properties of the infant-directed talk (IDT) register, which shows salient prosodic differences from adult-directed talk (ADT) in most languages. More information is needed regarding the articulatory/acoustic properties of IDT, and how these might relate to the infant's early language development. The experiments in this section will assess for developmental changes in the segmental and prosodic properties of IDT, and in infants' perception of them. The results of the proposed studies should converge to enhance our understanding of how the basic capacities for language that exist in young infants become realized as the infant acquires its native language. This knowledge is crucial for attempts to understand, diagnose, and intervene when language development does not proceed normally. This research plan is a direct outgrowth of the candidate's immediate career goal of improving the scientific understanding of the infant's transition to a speaker/hearer of its native language, and is consistent with her long-range goal of advancing knowledge about the biological bases of human language. A Research Career Development Award will strongly enhance the candidate's development as an independent investigator, by enabling her to devote the needed time to pursue the recently-developed research directions outlined here, and to develop related skills in the areas of early language acquisition and linguistic phonetics/phonology. Her department has provided her with excellent space, equipment, and student help to conduct the proposed research. In addition, the department would release her from the majority of her standard annual teaching and administrative responsibilities.